1. Field of the invention
The present invention concerns digital telecommunications. It is more particularly concerned with digital telecommunication systems in which the multiplexed digital bit streams transmitted are obtained by synchronous time-division multiplexing of digital tributaries at different bit rates according to a synchronous multiplexing hierarchy such as that specified in CCITT Recommendations G.707, G.708 and G.709.
2. Description of the Related art
The principle of this kind of multiplexing hierarchy is outlined in FIG. 1. The bit rates that can be multiplexed using this hierarchy are the bit rates standardized by the CCITT and shown in the righthand part of the figure: 2 048 kbit/s, 8 448 kbit/s, 34 368 kbit/s, 1 544 kbit/s, 6 312 kbit/s, 44 736 kbit/s, and 139 264 kbit/s.
There are various possible multiplexing structures for this multiplexing hierarchy depending on the bit rate of the tributaries to be multiplexed for a given application, and each multiplexing structure, such as that shown in bold line in the figure, corresponding to tributaries to be multiplexed with bit rates of 1 554 kbit/s, 2 048 kbit/s, 8 448 kbit/s and 34 368 kbit/s, comprises a number of hierarchy levels designated N1, N2, N3 in the example in question, going from the righthand part of the figure towards the lefthand part, i.e. in the direction in which the frames are formed from the various tributaries.
Tributaries can be introduced at the various hierarchy levels of a multiplexing structure and comprise entities referred to hereinafter as containers and entities referred to hereinafter as multiplexing units.
In what follows the terms container and multiplexing unit are used generically for sequences of entities and for individual elements within the sequences.
The multiplexing units constituted at a given hierarchy level and designated TU or AU (TU11, TU12, TU22 for level N1, TU31 for level N2 and AU4 for level N3 in this example) are formed by adding to the containers constituted at the same hierarchy level signals for indexing and justifying these containers relative to these multiplexing units.
The containers constituted at a given hierarchy level and designated VC (VC11, VC12, VC22 for level N1, VC31 for level N2 and VC44 for level N3 in this example) are formed by adding service signals, either to multiplex signals resulting from the multiplexing of "n" multiplexing units constituted at a lower hierarchy level, or to so-called information signals sampled on a tributary introduced at the level n question, designated C (C11, C12, C22 for level N1 and C31 for level N2 in this example).
FIG. 2 is a schematic showing the formation of the various containers or multiplexing units in the case of the multiplexing structure taken previously as an example. A container VC4 constituted at level N3 is obtained by multiplexing signals from four multiplexing units TU31a, TU31b, TU31c, TU31d constituted at level N2.
Two of these multiplexing units (TU3a and TU31b) are formed from containers VC31a and VC31b in turn formed from 34 358 kbit/s tributaries C31a and C31b introduced at level N2.
The other two multiplexing units (TU31c and TU31d) are formed from containers VC31c and VC31d in turn formed from multiplexing units TUG22 constituted at level N1 and which merely multiplex multiplexing units already constituted at the same hierarchy level, without adding indexing and justification signals.
The container VC31c is formed from four multiplexing units TUG22a, TUG22b, TUG22c, TUG22d in turn formed from four multiplexing units TU22a, TU22b, TU22c, TU22d, in turn formed from four containers VC22a, VC22b, VC22c, VC22d in turn formed from four 8 448 kbit/s tributaries C22a, C22b, C22c, C22d. The container VC31d is formed by multiplexing four multiplexing units TUG22e, TUG22f, TUG22g, TUG22h of which the first two (TUG22e and TUG22f) are formed like the multiplexing units TUG22a, TUG22b, TUG22c, TUG22d from 8 448 kbit/s tributaries C22e and C22f.
The third multiplexing unit TUG22g is formed from five multiplexing units TU11a, TU11b, TU11c, TU11d, TU11e respectively formed from containers VC11a, VC11b, VC11c, VC11d, VC11e in turn formed from five respective 1 544 kbit/s tributaries C11a, C11b, C11c, C11d, C11e.
The fourth multiplexing unit TUG22h is formed from four multiplexing units TU12a, TU12b, TU12c, TU12d respectively formed from containers VC12a, VC12b, VC12c, VC12d in turn formed from respective 2 048 kbit/s tributaries C12a, C12b, C12c, C12d.
The multiplexing unit constituted at the highest hierarchy level, which is the multiplexing unit AU4 in this example, is obtained by adding justification and indexing signals to the container constituted at this level, which is the container VC4 in this example.
The resulting STM frames are obtained by adding service signals to the multiplexing units constituted at the highest hierarchy level.
The diversity of the bit rates of the tributaries which form the frames resulting from such synchronous hierarchical multiplexing is reflected in the fact that the tributaries have within the resulting frames different information signal repetition periods, each of these periods being inversely proportional to the bit rate of the tributary. This repetition period is obtained by forming the product of the multiplexing factors "n" encountered all along the multiplexing structure for the tributary concerned. To give an example, the repetition period for the 2 048 kbit/s tributaries C12 is 64, that for the 1 544 kbit/s tributaries C11 is 80, that for the 8 448 kbit/s tributaries C22 is 16, and that for the 34 368 kbit/s tributaries C31 is 4.
The justification signals added to containers at a given hierarchy level to constitute multiplexing units provide for adapting the timing of the signals forming the containers to the timing of a local clock used at this hierarchy level, using the known positive-negative justification technique whereby a signal of a container is periodically substituted for a stuff signal provided for this purpose in the multiplexing unit formed from this container if the former timing is faster than the latter timing and a stuff signal is substituted periodically for a container signal if the former timing is slower than the latter timing.
The indexing signals produced at the various hierarchy levels serve to distribute to containers of lower levels the justification operations applied to containers of higher levels, to allow for the synchronous multiplexing effected at the various levels of the multiplexing hierarchy. In particular, they make it possible to situate each container constituted at a particular hierarchy level relative to the corresponding multiplexing unit constituted at this level, allowing for justification operations applied to this container for a given frame and for earlier frames. Also, they have a specific position within the corresponding multiplexing unit and consequently within the corresponding container constituted at the next higher hierarchy level, which (by successive recourse to the indexing signals produced at the various hierarchy levels encountered on running through the multiplexing structure in the direction opposite the direction in which the frames are formed from the tributaries) makes it possible to identify the container in question within the frames
The service signals added to the multiplexing units constituted at the highest hierarchy level in order to constitute the frames are located at repetitive positions within these frames, leading to the conventional representation of these frames in the form of tables or matrices having in practice nine lines numbered 0 through 8 and 270 columns numbered 0 through 269, reading from left to right and from top to bottom, that is say, line by line, each intersection between a line and a column representing a signal (a service signal, a justification signal, an indexing signal or an information signal) consisting in practice of one byte.
FIG. 3 shows a frame of this kind in the case of the example previously discussed where the highest hierarchy level is the level N3.
The shaded area in FIG. 3 contains the service signals SOH added to a multiplexing unit AU4 to constitute a frame and the unshaded area contains a multiplexing unit AU4.
A multiplexing unit AU4 is made up of a container VC4 to which are added indexing signals H1VC4 and H2VC4 which are always present and justification signals of which the signals H30VC4, H31VC4 and H32VC4 are always present except in the case of negative justification and of which the others (no reference symbols) are present only in the case of positive justification. The indexing signals H1VC4 and H2VC4 and, when they are present, the justification signals H30VC4, H31VC4 and H32VC4, respectively occupy columns 0, 3, 6, 7 and 8 of line 3; when present, the positive justification signals occupy columns 9, 10 and 11 of line 3.
Indexing signals H1VC4 and H2VC4 identify a container VC4 within a multiplexing unit AU4 and therefore within a frame, in practice by identifying the first byte of the container VC4, marked .DELTA. in FIG. 3.
FIG. 4 shows the position of a container VC4 within a given frame "m" and the next frame "m+1" (into which it overlaps by the very nature of the indexing signals and by virtue of the location of these indexing signals in line 3 of the frames, as shown in FIG. 3), the space occupied by the container VC4 being shaded. The content of a container VC4 is represented in FIG. 5 in the form of a table with nine lines and 261 columns, also read from left to right and from top to bottom; if there is no justification of the container VC4 relative to the multiplexing unit AU4, this table fits perfectly into the frame shown in dashed outline in FIG. 4, formed by the bytes in columns 9 through 269 of lines 3 through 8 of frame "m" and 0 through 2 of frame "m+1".
In practice the shape of the container VC4 departs from this nominal shape because of positive or negative justification applied to the container for earlier frames and up to the current frame "m", represented by a shifting of the first byte of the container VC4 (indicated by the bytes H1VC4 and H2VC4 of frame "m") and because of any justification applied to the container for frame "m+1". FIG. 4 shows the case where positive justification is applied to the container for frame "m+1", which justification (indicated by the bytes H1VC4 and H2VC4 of frame "m+1") is reflected in the insertion of stuff bits in columns 9 through 11 in line 3 of frame "m+1".
In the case of negative justification applied to frame "m+1", again indicated by bytes H1VC4 and H2VC4 of frame "m+1", container VC4 would not have, as shown in FIG. 4, a part indented by three bytes in line 3 of frame "m+1" but would protrude by three bytes on this same line at the level of columns 6 through 8, this negative justification being applied by setting the bytes of VC4 at the location of bytes H30VC4, H31VC4 and H32VC4 (negative justification opportunity bytes of frame "m+1").
The container VC4 in question is formed by multiplexing four multiplexing units TU31a, TU31b, TU31c, TU31d occupying the unshaded area in FIG. 5 and by adding service signals POHVC4 occupying the shaded area, that is to say the first or lefthand column of the table with nine lines and 261 columns. Each multiplexing unit (TU31a, for example) is in turn formed by adding to a container (VC31a in this example) indexing signals H1VC31a and H2VC31a and justification signals of which one (H3VC31a) is provided to give a negative justification opportunity and is always present except in the case of negative justification; another (not shown) is present only in the case of positive justification. The indexing and justification signals of the four VC31 containers are at a specific position relative to the first byte of the container VC4 and can therefore be identified once the latter has been identified, so that these containers can be identified, in practice by identifying the location of the first byte, respectively designated .DELTA.a, .DELTA.b, .DELTA.c, .DELTA.d.
The various VC31 containers (VC31a, VC31b, VC31c, VC31d) are also shown in FIG. 6, again in the context of the multiplexing structure being discussed by way of example, each of them being formed by adding service signals POHVC31a, POHVC31b, POHVC31c, POHVC31d either to multiplexed multiplexing units TUG22 or to signals from a tributary C31, as appropriate. Each of the VC31 containers can be represented, as shown in FIG. 6, in the form of a table read from left to right and from top to bottom, comprising nine lines and 65 (=260/4) columns, of which the first, which contains the service signals, is incomplete; the number of signals needed to complete it is equal to the number of indexing and justification signals attached to each VC31 container in the absence of positive and negative justification to constitute the corresponding TU31 multiplexing unit.
It would be possible to show the containers of lower hierarchy levels in a similar way, in other words in the form of a table having nine lines and a number of columns depending on the hierarchy level, and decreasing with the hierarchy level, some columns being incomplete.
Because of the indexing and justification operations applied successively to the various hierarchy levels the position within the frames of signals constituting given containers is not predetermined but can be determined, ignoring for the moment the complexity of the resulting processing, from the indexing signals of the containers concerned and of the containers of higher hierarchy levels.
For the same reasons, and because of the insertion of indexing, justification and service bytes within the frame, and because of the correlation between the number of elementary locations per lines that can be occupied by signals constituting a highest hierarchy level container and the number of multiplexing units of the next lower level multiplexed to form this highest level container, and because of the correlation between the number of justification bytes used at each hierarchy level and the number of multiplexing units that can be multiplexed at this level, the elementary locations assigned to signals constituting given containers are not reproducible from one line to the next of the frames, which is a very important drawback for equipment for processing such digital bit streams in the form of containers.
One object of the present invention is an interface for restructuring frames for such equipments enabling these disadvantages to be avoided.